Best answer by Mohammed
Do you mean for how long at any time you can Roam for?
If so, the Fair Usage Policy information is found here: https://www.idmobile.co.uk/help-and-advice/eu-roaming/fair-usage-policy
To quote the relevant part:
"But if you roam more than you use your phone at home (we mean in the UK) over a 4-month period, we might think you’re permanently roaming. If so, we'll take the following action:....."
Hope this helps.